1. Field of the Invention
The present invention relates to an electrophotographic photoreceptor (hereunder also called simply a “photoreceptor”) for use in electrophotographic printers, copiers, fax machines and the like, and to a manufacturing method therefore, and relates particularly to an electrophotographic photoreceptor having excellent printing durability and gas resistance due to additive improvements, and to a manufacturing method therefore.
2. Background of the Related Art
In general, an electrophotographic photoreceptor must have the function of holding a surface charge in the dark, the function of generating charge in response to received light, and similarly the function of transporting charge in response to received light. Such photoreceptors include monolayer photoreceptors that are provided with a monolayer photosensitive layer having all these functions, and stacked photoreceptors that are provided with a photosensitive layer comprising a stack of functionally discrete layers: primarily, a layer that contributes to the function of charge generation and a layer that contributes to the functions of holding a surface charge in the dark and transporting charge during photoreception.
The Carlson process for example may be applied to image formation by electrophotography using these electrophotographic photoreceptors. In this process, images are formed by charging the photoreceptor in a dark place, forming an electrostatic image of characters, pictures or the like from an original document on the surface of the charged photoreceptor, developing the formed electrostatic image with toner, and transferring the developed toner image to a support such as paper. After toner image transfer, residual toner and charge are removed from the photoreceptor, which can then be re-used.
Selenium, selenium alloys, zinc oxide, and cadmium sulfide and other inorganic photoconductive materials dispersed in resin binders and poly-N-vinyl carbazole, 9,10-anthracene diol polyester, pyrazoline, hydrazone, stilbene, butadiene, benzidine, phthalocyanine, and bisazo compounds and other organic photoconductive materials dispersed in resin binders and vacuum deposits and sublimates of these may be used as materials of such electrophotographic photoreceptors.
In recent years, with increased printing loads due to office networking and with rapid development of quick printing machines using electrophotography, electrophotographic printers are being required to have even greater durability and sensitivity as well as more rapid responsiveness. There is also strong demand for reductions in the effects of ozone, Nox and other gases generated within these devices, and for reduced fluctuation in image properties and the like due to fluctuations in the usage environment (room temperature, humidity).
At present, however, conventional photoreceptors do not always fulfill all the necessary requirements, and problems such as the following have arisen.
For example, the following problems arise with respect to wear resistance. In recent years, high-speed printing has become the norm due to the introduction of tandem imaging systems and the like not only in black-and-white printers and copiers, but also in machines that perform color printing. In addition to high resolution, precise image positioning has become an important requirement specification in recent years for color printers in particular. As more pages are printed, the surface of the photoreceptor becomes worn by friction with the paper and the various rollers, blades and the like, and when the degree of wear is large it becomes difficult to print images with a high degree of resolution and highly precise image positioning. There have already been a variety of studies aimed at improving wear resistance, but these have not been satisfactory.
Ozone is well known among the gasses produced within the devices. Ozone is produced by roller chargers and charging mechanisms using corona discharge, and it is thought that when the photoreceptor is exposed to ozone that remains or accumulates inside the device, the organic substances making up the photoreceptor become oxidized and their intrinsic structures are broken down, greatly degrading the properties of the photoreceptor. Nox is also produced when nitrogen in the air is oxidized by ozone, and it is thought that this Nox also degrades the organic substances making up the photoreceptor.
Such property degradation caused by gasses is thought to involve not only damage to the outermost layer of the photoreceptor, but also adverse effects that occur when gasses penetrate the inside of the photosensitive layer. It is believed that the outermost layer of the photoreceptor is itself whittled away to a greater or lesser extent by friction with the various other members discussed above, but when harmful gas penetrates the inside of the photosensitive layer, the structures of the organic substances within the photosensitive layer may be damaged, so controlling such penetration of harmful gasses is an issue. In tandem-type color electrophotographic devices using multiple photoreceptors in particular, one obstacle to production of satisfactory images is thought to be the variations in hue that occur when there are differences in the degree of effect of gasses according to the positioning of the drums inside the device and the like. Thus, property degradation due to gasses is a particularly important problem in tandem-type color electrophotographic devices.
In Patent Document 1 and Patent Document 2, hindered phenol compounds, phosphorus compounds, sulfur compounds, amine compounds, hindered amine compounds and other antioxidants are used for improving gas resistance. Patent Document 3 proposes a technique using a carbonyl compound, while Patent Document 4 proposes a technique using a benzoate compound or salicylate ester compound. Techniques for improving gas resistance are also proposed using a specific polycarbonate resin together within biphenyl and other additives in Patent Document 5, using a combination of a specific amine compound and a polyarylate resin in Patent Document 6, and using a combination of a polyarylate resin and a compound with a specific light absorbency in Patent Document 7. With these techniques, however, either the resulting photoreceptor is not sufficiently gas resistant, or even if the gas resistance properties are satisfactory wear resistance is not improved, and satisfactory results have also not been obtained for other properties (image memory, potential stability during printing endurance and the like).
Patent Document 8 shows that the effects on the photoreceptor of gas occurring around the charging mechanism can be controlled by keeping the oxygen gas permeability coefficient of the surface layer below a certain level when combined with a charge transport layer having a particular degree of charge mobility. Meanwhile, Patent Document 9 shows that wear resistance and gas resistance can be improved by keeping steam penetration of the photosensitive layer below a specific value, but with this technique the desired effects can only be obtained using a specific polymer charge transport material, and because of limitations on the structure and mobility of the charge transport material, this is not compatible with a the demands of various electrical properties.
According to Patent Document 10, a monolayer electrophotographic photoreceptor with superior gas resistance can be obtained using a specific diester compound with a melting point of 40° C. or less in the photosensitive layer. However, when a substance with a low melting point is added to this layer and the photoreceptor containing the substance is in long-term contact with a cartridge or parts of a device body, satisfactory results may not be obtained because the compound seeps into the contacting parts in a phenomenon called bleeding, causing image defects.
Regarding property fluctuation of the photoreceptor according to usage environment, one example is image property degradation in low-temperature, low-humidity environments. That is, in general the apparent sensitivity properties and the like of the photoreceptor are reduced in low-temperature, low-humidity environments, resulting in obvious deterioration in image quality (loss of image concentration, tone loss in halftone images). Image memory resulting from reduction sensitivity properties may become conspicuous. This occurs because during printing, an image recorded as a latent image in the first drum rotation is also affected by potential fluctuations during the second and subsequent drum rotations, and when printing halftone images in particular, the image deteriorates because printing occurs in unwanted areas. Negative memory, in which the concentration of the printed image is reversed, is often conspicuous in low-temperature, low-humidity environments.
Another example is image property degradation in high-temperature, high-humidity environments. That is, in general the transfer speed of charge in the photosensitive layer is higher in high-temperature, high-humidity environments than at normal temperature and humidity, and problems of excessive increase in printing concentration and tiny black spots (fogging) in solid white images are observed as a result. An excessive increase in printing concentration causes increased toner consumption, and tone gradations are also lost because the diameter of each dot is increased. In terms of image memory, in the reverse of the situation in low-temperature, low-humidity environments, there is often a conspicuous problem of positive memory, in which the concentration of the printed image is reproduced as is.
Such property degradation due to temperature and humidity conditions is commonly caused by the absorption or release of moisture from the charge generating material or the resin binder in the surface layer of the photosensitive layer. Various means have already been studied to deal with this, such as by adding specific compounds to the charge-generating layer as in Patent Document 11 and Patent Document 12, or by using a specific polycarbonate polymer charge transport material in the surface layer as in Patent Document 13, but as yet no satisfactory material has been discovered having the necessary properties for controlling the effects of temperature and humidity on the photoreceptor and the like.
The technique disclosed in Patent Document 14 can solve the problem of property degradation caused by temperature and humidity as discussed above, but is not necessarily satisfactory in terms of wear resistance. Moreover, although Patent Document 15 discloses a diallyl adamantane dicarboxylic acid useful as a raw material of a resin that can be used as an optical material or electrical material, compounds having adamantane structures have not been sufficiently studied as additives for photoreceptors. In addition, Patent Document 16 discloses a photoresist composition containing a compound having an adamantane structure.
Patent Document 17 discloses a carboxylic acid derivative having an adamantane structure, while Patent Document 18 discloses a novel adamantane carboxylic acid ester compound, and Patent Documents 19 and 20 disclose methods of synthesizing diadamantyl diester compounds, but the use of these compounds as additives for photoreceptors is not adequately addressed in any of these documents.
Patent Document referred to in the foregoing include:
Patent Document 1: Japanese Patent Application Publication No. S57-122444;
Patent Document 2: Japanese Patent Application Publication No. S63-18355;
Patent Document 3: Japanese Patent Application Publication No. 2002-268250;
Patent Document 4: Japanese Patent Application Publication No. 2002-287388;
Patent Document 5: Japanese Patent Application Publication No. H6-75394;
Patent Document 6: Japanese Patent Application Publication No. 2004-199051;
Patent Document 7: Japanese Patent Application Publication No. 2004-206109;
Patent Document 8: Japanese Patent Application Publication No. H08-272126;
Patent Document 9: Japanese Patent Application Publication No. H11-288113;
Patent Document 10: Japanese Patent Application Publication No. 2004-226637;
Patent Document 11: Japanese Patent Application Publication No. H6-118678;
Patent Document 12: Japanese Patent Application Publication No. H7-168381;
Patent Document 13: Japanese Patent Application Publication No. 2001-13708;
Patent Document 14: Japanese Patent Application Publication No. 2007-279446;
Patent Document 15: Japanese Patent Application Publication No. S60-100537;
Patent Document 16: Japanese Patent Application Publication No. H9-265177;
Patent Document 17: Japanese Patent Application Publication No. 2001-39928;
Patent Document 18: Japanese Patent Application Publication No. 2003-306469;
Patent Document 19: U.S. Pat. No. 3,342,880; and
Patent Document 20: Japanese Patent Application Publication No. S48-10055.
As discussed above, various techniques have been proposed in the past for improving photoreceptors. However, while the techniques described in these patent documents are satisfactory in terms of wear resistance and the properties of the photoreceptors, they are not able to sufficiently control the adverse effects of harmful gasses and environmental temperature and humidity on the photoreceptor, and further improvements are needed.
It is therefore an object of the present invention to provide an electrophotographic photoreceptor having sufficient wear resistance and satisfactory characteristics as a photoreceptor, while being little affected by harmful gasses and environmental temperature and humidity.